The present invention relates generally to optical measurement systems. More specifically, the present invention relates to the measurement of features in semiconductor manufacturing.
The successful manufacture of advanced sub-micron sized semiconductor devices requires the detection, measurement and evaluation of defects and imperfections as small as 1 micron on the photographic mask (photomask) used to pattern the wafer. Defect inspection and measurement techniques for masks therefore play an important role in mask making and quality assurance. Measurement of line widths is also important in producing an acceptable mask.
Thus, it is becoming increasingly important to be able to identify and to correctly size mask defects, line widths, and other features that are under 1 micron in size. Accurate sizing of these features allows masks that are below specification to be repaired, and prevents the needless and costly hold up of masks that do meet specification. However, one of the problems of assessing reticle quality at these sub-micron levels on an automatic inspection system is that the size of these features cannot always be accurately, quickly and cost-effectively measured in a production environment.
It has long been known that mask inspection tools are not measurement tools and that the size information provided by these tools has limited value. Consequently, many mask makers have incorporated measurement aids at the inspection station or move the mask to a more suitable measurement tool in order to make classification decisions. Measurement aids used at the inspection station include calipers, grids, and software based video image markers such as gates, scales, grids, boxes and circles. These aids are fairly rapid, but ultimately require the operator to xe2x80x9ceyeballxe2x80x9d the boundaries of the defect. This activity is very subjective and can lead to an error in the measurement of the defect.
For example, feature size is often measured by measuring the distance between opposite edges of the feature. Once a feature is identified by an inspection machine, the operator uses a video microscope and a television camera to position a cursor on one side of the feature and another cursor on the other side of the feature. The operator must judge for himself the exact boundaries of the feature and must place the cursors where he sees fit. At this point, the operator pushes a button and the software blindly computes the distance between the two cursors in order to supply a rough approximation of the dimension of the feature. This technique has many disadvantages.
Firstly, this measurement technique is operator dependent in that the operator must manually position the cursors on the boundaries of what the operator believes to be the feature. The operator may misjudge the type of a feature, its boundaries, or may simply misplace a cursor even if the feature is visible. The software then calculates the distance between the cursors, without regard for the type of feature, its true boundaries, etc. The above technique may be performed with a standard video microscope and has an accuracy of about 0.1 micron, but is completely subject to the operator""s skill level and interpretation.
Another difficulty with light measurements of features less than 1 micron in size is that the wavelength of photons begins to interfere with the measurement of these 1 micron and less feature sizes. Many techniques do not adequately address the non-linearities associated with such measurements.
Alternatively, the mask may be removed from the automatic inspection tool and relocated on a more precise and repeatable measurement tool. However, this approach involves removing the mask from production, relocating the feature, and is thus impractical in a production environment. This technique is also costly, time-consuming and increases the handling risk. For example, an atomic force microscope (AFM) may be used to measure feature sizes; such a microscope is extremely accurate but is very slow, very expensive and is still subject to operator interpretation.
One approach that has been taken that uses calibration of an automatic inspection system in order to size defects is described in Characterization Of Defect Sizing On An Automatic Inspection Station, D. Stocker, B. Martin and J. Browne, Photomask Technology and Management (1993). One disadvantage with the approach taken in this paper is that it only provides a technique for measurement of defects of 1.5 microns and greater. Such sizes of defects would produce a linear relationship between reference sizes and actual measured sizes, and the paper does not account for defects less than 1 micron that would produce a non-linear relationship.
In general, measurement of line widths on a semiconductor mask can be broken into the two categories of isolated lines and dense lines. U.S. Pat. No. 5,966,677 describes a useful flux-area technique for measuring isolated defects or line widths, especially when they are smaller than the wavelength of light used to view them. This technique works best when at least 1.5 blur distances (the blur distance equals the wavelength in diffraction-limited optics) separates lines and/or features. Unfortunately, lines are not always separated by such distances.
The category of dense lines refers to adjacent lines (or other features) where the distance between the lines is less than about 1.5 times the wavelength being used to measure the lines. Prior art techniques for measuring line widths of dense lines are not always accurate or desirable.
In particular, line width measurement (also known as critical dimension or CD measurement) suffers increasingly from two basic limitations as feature sizes shrink. Firstly, features that are smaller then the wavelength of light become very hard to measure because their edge-to-edge measurement appears to approach the wavelength of the light used to examine them. Thus, measurement of line widths that are about the wavelength of light is difficult using prior art techniques. Secondly, measurement of a line width that has an adjoining feature (a line or other feature) that is less then about one and one-half times the wavelength of the examining radiation in distance becomes difficult. The defracted light from the neighboring feature confuses the measurement of the line width. Usually the confused measurement will be smaller then it should be because the neighboring feature will appear to increase the background light level.
FIGS. 1 and 2 illustrate problems with the conventional full-width half-maximum technique for measuring widths of dense lines.
A phenomenon is known as the optical proximity effect (OPE) influences critical dimension measurements of photographic mask and wafers in semiconductor manufacturing and has been a subject of intense interest and investigation for many years. OPE is caused by the convolution of the intensity profiles of adjacent lines and introduces errors in the determination of the line edge positions and in turn the line width. OPE is the result of any optical imaging system where adjacent line edges spread spatially such that the tail of the adjacent edge spread function if sufficiently close, influences the shape and thereby the determination of the position of the line edge in question. If the line edges for the line in question cannot be determined accurately, the line width cannot be measured accurately.
FIG. 1 shows a prior art approach which uses the full-width half-maximum technique to determine line width where the lines are isolated. Graph 10 is an intensity profile for an isolated line. Graph 10 includes an intensity axis 12 and a width axis 14 in microns. Intensity profile 16 is a profile for an isolated line such as is present on a photographic mask that is desired to be measured. Taking the 50% threshold values 18 and 20 for profile 16 easily leads to a calculation for the width of the line to be about 1 micron. For such an isolated line, measurement of its width based upon its intensity profile is relatively straightforward and accurate.
FIG. 2 is a graph 20 of intensity profiles for a line having an adjacent edge. Graph 20 includes an intensity axis 12 and a width axis 14 in microns. Shown superimposed in the graph are the resulting intensity profile 16xe2x80x2 for the same line to be measured as in FIG. 1 and the intensity profile 22 of an adjacent edge in proximity. Due to the proximity of the adjacent edge (as indicated by the proximity of its profile 22), the profile 16xe2x80x2 for the line has a different shape on its right hand side. Most notable, while the 50% threshold 18 is the same for the line, the 50% threshold for profile 16xe2x80x2 occurs at location 24 rather than location 20 in FIG. 1. Thus, using the thresholds to measure the width of the line results in a measurement of about 0.9 microns rather than 1 micron which is the true width of the line. The resulting shift in the line width (as defined by the distance between edges at the 50% threshold level) results in a line that appears to be thinner. Note that this result could be caused by an adjacent edge or also by an adjacent line or any other adjacent feature that is close enough in proximity to the line to be measured to cause convolution of the intensity profiles. Thus, such an adjacent edge or other feature causes an inaccurate measurement of a line width. Further, the resultant profile 16xe2x80x2 of FIG. 2 does not have a flat baseline that can be used as a reference. Thus, it is difficult to attempt to measure the flux under the curve to determine the area and thus the line width.
Prior art techniques for measuring line widths of dense lines are not always accurate or desirable. In a first conventional technique used for dense lines, a line width is measured by first measuring the distance between adjacent lines and then by applying a correction. This technique is referred to as multipoint calibration (MPC) and is not extremely desirable because it is hard to calibrate, and is highly dependent on the optical system used. A second conventional assumes that lines have uniform width and spacing. The average flux and pitch of the lines are measured, the fill ratio determined, and thereby the width of the clear and opaque regions are determined. This technique is not extremely desirable because lines of uniform width and spacing are seldom measured.
The paper entitled Optical Proximity Effects in Sub-micron Photomask CD Metrology by Doe et al. presents an extended multipoint calibration (EMPC) technique for line width measurement amongst densely packed lines. In particular, this paper discusses use of the threshold and maximum gradient algorithms for critical dimension calculations. These techniques are not always desirable because it is hard to calibrate, is highly dependent on the optical system used, and performs poorly when lines are very close. Thus, prior art techniques do not adequately address measurement of densely packed lines.
Turning now to a discussion of opacity, for isolated features it is desirable to be able to determine the opacity of the feature to be used in determining a corrected dimension (close to the actual dimension). For example, a dimension of a feature may be measured using the flux-area measurement technique described in U.S. Pat. No. 5,966,677 referenced above. The flux-area technique provides the effective width of the feature, which indicates that the feature absorbs the same amount of flux as a chrome feature having a width equivalent to the effective width. If the feature is less than opaque, though, its true width will be greater than the effective width. Thus, if a measurement of the actual width is desired, the effective width needs to be corrected. Other techniques for measuring a feature width may also result in values that should be corrected if a closer estimate of the actual width is desired.
By determining the opacity of a feature, its dimension can be corrected to determine a more accurate value. Although techniques for determining the opacity of a feature have been previously been described in U.S. patent application Ser. No. 09/028,207 referenced above, further techniques are desirable to improve upon those already described. It is desirable to have improved techniques in situations where quick set up and execution times are desirable.
Turning now to a discussion of irregular feature measurement, it is also desirable to be able to measure the height and width of a feature which may not be circular. That is, the feature may be irregular or have an oval shape. Although the diameter of a feature can be measured using the flux-area technique of U.S. Pat. No. 5,966,677, this technique assumes that the feature is roughly circular. Dimensions of features that are not necessarily circular are desirable. Isolated features whose dimensions are approximately larger than the wavelength of the radiation being used (about 0.5 microns for visible light) for measurement can be measured using conventional measurement techniques that are well-known in the prior art. For an isolated non-circular feature whose dimensions are approximately the size of the wavelength being used or smaller, its height and width may be measured as described in U.S. patent application Ser. No. 09/028,207 referenced above. Although useful, further techniques are desirable to improve upon those already described. It is desirable to have improved techniques in situations where quick set up and execution times are desirable.
Therefore, a feature measurement system is desirable that can provide reliable and repeatable measurements of densely packed lines and other features that are closer than about 1.5 times the wavelength being used for the measurement. It is also desirable for such a system to be able to measure opacity and height/width for features with greater speed. It would be especially desirable for such a system to operate in a fast and highly practical manner in a production environment.
A first embodiment of the present invention extends the flux-area technique by allowing line widths to be measured accurately even when the lines to be measured are closer than 1.5 times the wavelength being used. The lines may be as close as {fraction (1/10)} the wavelength being used, or closer if camera and digitizer noise is sufficiently low. This embodiment can also be used for measuring any of a variety of other features that are densely packed, and can measure widths of lines which are extremely close to features other than lines.
In this first embodiment a simulated intensity profile is generated based upon estimated edge positions. The simulated profile is then subtracted from the actual measured intensity profile to obtain an error profile. The magnitude of the error for each edge determined from the error profile provides edge position corrections which are used to adjust the originally estimated edge positions. A new simulated intensity profile is then created from the newly estimated edge positions and the process is repeated until the error profile is deemed acceptable. Once deemed acceptable, the line width may be measured simply by subtracting the estimated edge positions of the line of interest.
Alternatively, line width may be measured by generating an isolated intensity profile for the line of interest. To generate this intensity profile, intensity profiles for the interfering edges are created using the final estimated edge positions. The simulated edge profiles are added together to produce a simulated interfering edge profile which is then subtracted from the original, measured intensity profile thereby leaving the isolated profile of the line of interest. The flux-area technique described in U.S. Pat. No. 5,966,677 referenced above may then be used to determined line width from the intensity profile.
A second embodiment is used to determine the opacity of a feature to use in correcting diameter or width measurements as well as helping to determine of what the feature is made. While prior art techniques may produce opacity values for relatively large particles, the present invention is especially useful for dimension measurements of less than perfectly opaque features that are less than about the wavelength of the examining radiation in size.
Advantageously, use of a single data point for contrast versus diameter to develop opacity data in the second embodiment allows for a rapid set up time for the system and quicker measurements. An operator can set up the system and perform calibration in approximately one minute (as compared to ten minutes in the prior art), and measurements can be made in approximately one second.
In this second embodiment, if one can assume a priori that the opacity is 100% then measurement of contrast can be used to determine the diameter of an isolated defect (or the width of an isolated line). It is realized that sizes that are smaller than about the wavelength of the examining radiation have a simple one-to-one relationship with the measured contrast of the feature. Thus, measuring the maximum contrast of such a feature can be used to calculate the defect diameter or line width.
In a third embodiment, peak width data is used to solve for the height or width of a feature without using the flux-area technique. Thus, more accurate dimensions for irregular features can be determined without having to assume that a feature is circular. It should be appreciated that the dimensions of height and width are arbitrarily imposed on the orientation of a two-dimensional feature, and the two terms can be interchanged without effecting the operation of the present invention. As used herein, width refers to a dimension parallel to a nearby line, while height refers to the dimension orthogonal to width.
Further, the second and third embodiments for determining opacity and the width/height of a feature may also be used with densely packed features. The technique of the first embodiment is first used to generate an isolated intensity profile for the feature of interest, then the second and/or third embodiments use the generated intensity profile for calculation of opacity or width/height.
Thus, embodiments of the present invention disclose a feature measurement system that provides an objective, practical and fast method for accurate measurement of characteristics of photographic masks. The present invention may be used while the mask is in place at the inspection station; there is no need for the mask to be removed to a different machine for measurement. A characteristic of the feature (such as diameter, width or opacity) is quickly measured by the present invention.
Benefits include avoiding repairing masks within specification and reaching equivalent results whether measured by customer or supplier (when calibrated with the same reference). Operator productivity and tool utilization is improved by rapid measurements taking place at the inspection station. Thus, by providing an extremely accurate measurement of mask features, the disclosed measurement tool helps to avoid unnecessary mask repairs and allows for improved process control. Also, operator variability is eliminated, and overall productivity and mask throughput is increased due to the accurate measurements in-place and documentation produced in seconds. Because the measurements are automatic, operator training is minimal.